Helical bar strip chart recorder

ABSTRACT

In a strip chart recorder, the graph of the signal to be recorded is generated by a helical bar printer comprising a helical bar of several convolutions with a blade hammer positioned to strike each convolution of the helical bar. The analog value to be recorded is converted into a digital representation with a radix equal to the number of convolutions of the helical bar. One of the hammers is selected in accordance with the value of the digital representation and the selected hammer is actuated to strike the bar to print a dot at a time corresponding to the difference between the value of the digital representation and the analog value being recorded to thus print the dot in a position corresponding to the analog value. The hammers are repeatedly actuated to effect the recording of a graph by the resulting pattern of printed dots.

Unite States Patent 1 Pear, Jr.

[ HELHCAL BAR STRIP CHART RECORDER [75] Inventor: Charles B. Pear, Jr., Greenlawn,

[73] Assignee: Potter Instrument Company lne., Plainview, NY.

22 Filed: Oct. 19, 1972 [21] Appl. No.: 298,834

[4 1 Oct. 22, 1974 Primary Examiner-Richard B. Wilkinson Assistant Examiner-Vit W. Miska Attorney, Agent, or Firm-Lane, Aitken, Dunner & Ziems l 5 7 1 ABSTRACT In a strip chart recorder, the graph of the signal to be recorded is generated by a helical bar printer comprising a helical bar of several convolutions with a blade hammer positioned to strike each convolution of the helical bar. The analog value to be recorded is converted into a digital representation with a radix equal to the number of convolutions of the helical bar. One of the hammers is selected in accordance with the value of the digital representation and the selected [56] References Cited hammer is actuated to strike the bar to print a dot at UNITED STATES PATENTS a time corresponding to the difference between the I value of the digital representation and the analog 3,214,764 10/136; mllim lii flr. value being recorded to thus print the dot in a position 1x 2 0 er 146/ corresponding to the analog value. The hammers are 2 x a repeatedly actuated to effect the recording of a graph h A by the resulting pattern of printed dots.

12 Claims, 4 Drawing Figures 55 57 71 75 a l w HAMMER DRIVERS FF SCALE RANGE SELECTO INT.

1 HELICAL BAR STRIP CHART RECORDER BACKGROUND OF THE INVENTION The best conventional strip chart recorders of the prior art, such as those employing pen and ink tech niques to generate the graph of the recorded value, are relatively slow in response time requiring at least 250 milliseconds to cover the full scale of the graph. This slow response time results from the inertia of the marking instrument which produces the graph. Instead of recording by means of a pen and ink marking instrument, it has been proposed in the prior art to generate the graph by means of pressure between a rotating helical bar and a pressure bar, such as used in facsimile reproduction, to produce the strip chart graph. These helical bar printers of the prior art suffer from reduced resolution without dramatically improving the recording response time. The present invention utilizes the helical bar principle to effect generation of the graph, but achieves a much faster full scale response time of 100 milliseconds or better. This greatly improved response time is achieved with a resolution comparable to that of the highest quality-strip chart recorders.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, the recording instrument is a helical bar printer comprising a rotatable drum on which is mounted a helical bar extending around the drum in convolutions. The drum and helical bar are rotated about their common axis at a constant speed. Adjacent to the drum and helical bar are 10 blade hammers, each spanning one convolution, and each adapted when actuated to strike the adjacent convolution of the helical bar. The strip chart on which the recording is to take place is positioned between the blade hammers and the helical bar, and is moved in a direction past the drum and hammers in a direction perpendicular to the axis of the drum. When one of the hammers is actuated, it will strike the adjacent helical bar through the strip chart and effect the printing of a dot on the strip chart. This printing is preferably achieved by an inking ribbon also positioned between the hammers and the helical bar, but may be achieved by using a strip chart comprising pressure sensitive paper. The scale of the graph to be recorded extends over all of the ten convolutions of the helical bar. The analog signal to be recorded is converted to a digital representation having a radix of 10 equal to the numberof hammers and convolutions of the helical bar. The hammer is selected in accordance with the value of the digital representation and then this hammer is actuated at a time corresponding to the difference between the analog value being recorded and the value of the digital representation. To record a dot at a position on the strip chart corresponding to the analog value, the strip chart moves continuously past the row of hammers and helical bar, and as the strip chart moves, the hammers are repeatedly actuated, as described above, to print dots to effect a graph of the analog signal printing one dot per revolution of the drum. The high response time together with the improved resolution is achieved because the range of the graph being recorded extends over several convolutions of the helical bar and several blade hammers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating the strip chart recorder of the present invention;

FIGS. 2-4 illustrate circuits of components of the system illustrated in FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT In the system of the invention, as shown in FIG. I, the analog input signal to be recorded is in the form of a voltage with a positive polarity. It is first applied to a range selector in which a selective attenuation may be applied to the input signal to thus select a range for the recording of the input signal. The positive input signal voltage, after being attenuated by the range selector 11, is applied to a series of voltage level detectors 2029, each of which detects whether the input signal is within a given range or increment relative to a reference signal voltage E .The voltage level detector 20 detects whether the input signal is within the range or increment from 0 to 0.1E and if so, producres an output signal which is applied to an E switch 31 and also enables a gate 40. The voltage level detector 21 determines whether the input signal is within the range or increment of 0.1E to 02E and if so, produces an output signal which is applied to the E switch 31 and also enables a gate 41. Similarly, the voltage level detector 22 determines whether the applied signal is within the range of 0.2E to 0.3Ek, and if so, enables a gate 43. The output signal of the voltage level detector 22 is also applied to the E switch 31. The voltage level detectors 23-29 function in a similar manner to detect whether the input signal falls in successively higher consecutive ranges, each one tenth of the reference voltage E in width. Thus, the voltage level detector 29 detects whether the input signal is within the range 0.9E to E Each of the voltage level detectors 23-29 is connected to enable a corresponding gate 4349 if the input signal falls within the voltage increment to which the detector is sensitive and is also connected to apply a signal to the E switch 31. The voltage level detector 29 will produce an output signal indicating that the applied input signal is off scale if the input signal is greater than E Thus, one of the voltage level detectors 20-29 will produce an output signal indicating within a tenth of the reference voltage E; where the input signal falls if the input signal is less than E and will enable a corresponding one of the output gates 4049. The voltage level detector will also apply a signal to the E switch 31 indicating in what increment the input signal falls.

The E; switch 31 in response to the applied signal from one of the voltage level detectors will produce an output signal equal to the lower limit of the increment in which the input signal falls as detected by the series of detectors 20-29. For example, if the input signal is within the range of 0.5 to 0.6, the voltage level detector 25 will produce an output signal enabling gate 45, and in response to this output signal, the E switch 31 will produce an output signal equal to 0.5E The output signal of the E switch 31, which has a negative polarity, is applied to a summing amplifier 51, which also receives the positive output signal directly from the range selector 1 1. Thus, the magnitude of the output signal of the E switch 31 is subtracted from the magnitude of the output signal of the range selector 11 by the summing amplifier 51, which produces an output signal voltage representing the difference in the magnitude between these two signals, or in other words, representing the amount that the magnitude of output signal of the range selector exceeds the magnitude of the output signal of the E switch 31. Thus,- th output'signal of the summing amplifier 51 represents the amount that the output signal of the range selector 1 1 exceeds the lower limit of the voltage incrementin which the signal falls as detected by one of the voltage level detectors 20-29. The output signal of the summing amplifier 51 is positive and is applied to a summing amplifier 53.

The-recording of the voltage signal is carriedout by a helical bar printer, such as that disclosed in copending application Ser. No. 284,274, entitled Helical Bar Printer Logic Circuitry, filed Aug. 28, 1972, invented by Henry P. Kilroy and Harold J. Murphy and assigned to the assignee of the present application. As disclosed in the above mentioned copending application, the printing mechanism comprises a drum 55, on a cylindrical surface of which is mounted a helical bar 57 making a plurality of convolutions about the drum. In the, system of the present invention, the helical bar makes l convolutions about the drum. Adjacent to each convolution of the helical bar 57 is a blade hammer which when actuated will strike the bar 57 and cause a dot to be printed on a strip chart (not shown) positioned between the hammers and the drum 55. Thus, each of the blade hammers and the opposite convolution of thehelical bar comprises a separate dot marker. The blade hammers are designated by the reference numbers 60-69. The actual printing may be carried out by providing an inking ribbon (not shown) between the drum 55 and the hammers or by making the strip chart'out of pressure sensitive paper. If desired, several copies may be printed simultaneously by the conventional carbon copy technique used with typewriters. 2 I

The drum 55 is continuously rotated by a motor 70 so that the lateral position of the dot printed by a given hammer depends upon the time that the hammer is actuated. The gross positioning of the dot is determined by the hammer that is actuated. Thus, the position that a dot is printed over the entire range of the strip chart can be controlled by selecting the hammer to be actuated, and then selecting the timeof actuation of the hammer. The hammer is selected in accordance with the voltage increment in which the output signal of the range selector 11 falls as detected by one of the voltage level detectors 20-29. The time of actuation of the hammer is determined by'the amount that the output signal voltage of the range selector ll exceeds the I lower limit of the voltage increment in which the signal falls as detected by one of the voltage level detectors 20-29. In this manner, the position of the dot printed on the strip chart between the drum 55 and the hammers 60-69 represents the amplitude of the output signal of the range selector 11. By repeatedly selecting and actuating one of the hammers 60-69 in accordance with this signal, a graph of the value of the signal is recorded.

The motor 70 is connected to drive with the drum 55 a tone wheel 71 which by means of a transducer 73 produces one hundred and forty output pulses for each revolution of the drum 55 at equal angular intervals of the drum 55. A second wheel 75 produces one output pulse by means of a transducer 77, each time the drum passes through the start position, which is the position at which each convolution of the helical bar 57 is opposite the extreme left-hand side of each of the hammers 60-69. The rotation of the drum 55 is in the direction in which continued rotation of the drum 55 from the start position will advance the intersection of the bar 57 with the adjacent hammers from left to right as viewed in FIG. 1. The output pulses produced by the transducer 73 are shaped by a pulse shaper 79 and applied to a linear gate 81 to enable it. A linear gate is a circuit which passes an analog signal voltage from its input to its output when enabled. The linear gate 81 has a voltage of 0.1E applied to its input with a negative polarity so that the linear gate 81 produces output pulses having an amplitude of 0. 1 E each time a pulse is produced by the transducer 73, or in other words, each time the drum 55 passes through one l/l40th of a revolution. The output signal pulses of the linear gate 81 are attentuated to one-tenth of their value by a voltage divider 83, and then are applied to an integrator 85 which integrates the applied pulses and produces an output signal voltage equal to the sum of the amplitudes of the applied input pulses from the time that the integrator 85 was last reset to zero. The integrator 85 is reset to zero each time the transducer 77 produces an output pulse when the drum 55 passes through the start position. Thus, the integrator 85 will produce an output signal voltage representing the number of pulses that have been produced by the transducer 73 since the last pulse was produced by the transducer 77 or since the drum 55 last passed through the start position. Accordingly, the output signal voltage of the integrator 85 will represent the angular position of the drum 55 with respectto the start position. Theoutput signal of the integrator 85 is applied to a voltage divider 87 which attenuates the output voltage to one 1/l4th of its value. The output of the attenuator 87 will thus be N/ X 0.lE where N is the number of pulses produced by the transducer 77 since the drum 55 last passed through the start position. The output signal voltage of the attenuator 87 is applied to the summing amplifier 53.

As pointed out above, the summing amplifier 53 is also connected to receive the output signal voltage of the summing amplifier 51, which is positive. Since the output signal voltage from the voltage divider 87 will be negative, the summing amplifier 53 will produce an output signal equal to the difference in magnitude between the two input signals. The output signal voltage of the summing amplifier 53 will be positive until the magnitude of the output signal voltage of the amplifier 51 is exceeded by the output signal voltage from the voltage divider 87 whereupon the output signal voltage of the summing amplifier 53 will become negative. The output signal voltage of the summing amplifier 53 is applied to a negative voltage detector 89, which will produce an output signal at the time that the output signal voltage of the summing amplifier 53 becomes negative. As pointed out above, the output signal voltage of the summing amplifier 51 represents the amount that the output signal of the rangeselector 11 exceeds the lower limit of the voltage increment in which the output signal falls as detected by one of the detectors 20-29. The summing amplifier 53 will become negative and cause the negative voltage detector 89-to produce an output signal at a time following the production of a start pulse by the transducer 77 corresponding to the output signal voltage of the summing amplifier 51. The angular position of the drum 55 relative to the start position at the time that the negative voltage detector 89 produces an output signal thus will correspond to the output signal voltage of the summing amplifier 51 and thus will correspond to the amount that the output signal of the range selector 11 exceeds the lower limit of the voltage increment in which the signal voltage falls as detected by one of the voltage level detectors -29.

The output signal produced by the negative voltage detector 89 is applied to the gates -49 and will pass through the enabled one of the gates to a corresponding one of the series of the hammer drivers 90-99, which then applies a firing pulse to the corresponding one of the hammers 60-69. In this manner, one of the hammers 60-69 will be selected to be fired corresponding to the voltage increment in which the voltage signal level falls as detected by one of the detectors 20-29 and the time that the hammer is fired will correspond to the amount that the signal voltage exceeds the lower limit of the voltage increment in which the signal falls. Thus, a dot will be printed in a position displaced from the left side of the strip chart as would be viewed in FIG. 1, corresponding to the amplitude of the output signal voltage of the range selector 11 at the proper time after each start pulse produced by the transducer 77. The strip chart on which the printing is being carried out is continuously advanced so that the series of dots printed in this manner will form a graph representing continuously the amplitude of the output signal voltage of the range selector l1 and in this manner, the system records the output signal voltage of the range selector 11. The speed of the strip chart is preferrably.

selected so that adjacent dots representing the same signal amplitude are substantially contiguous. Different ranges of amplitudes of the input signals applied to the range selector 11 are readily accommodated by changing the attenuation selected by the range selector 11.

The series of voltage level detectors 20-29 is actually an analog to digital converter which converts the output signal voltage of the range selector into a digital representation with a radix of ten. The hammer to be tired is selected in accordance with thevalue of this digital representation. The fact that the time of actuation of the'hammer is in response to a number of pulses produced can be considered to mean that the input analog signal is digitized into two digits, one digit selecting the hammer, and the second digit corresponding to the number of pulses selecting the time of actuation of the hammer. The radix of the two digits, however, is different, the first being ten corresponding to the number of hammers, and the second being one hundred and forty corresponding to the number of pulses produced per revolution of the drum 55.

The circuitry of the range selector 11 is illustrated in FIG. 2. As shown in FIG. 2, the analog input signal is applied across a voltage divider 101 having a plurality of tabs which are connected to different contacts 103 of a range selector switch. The range selector switch has a movable pole 105, which can selectively connect the output of the range selector to any of the contacts 103, as shown. One of the contacts 103 provides a direct connection of the input signal to the output of the range selector. By moving the pole 105 to the desired contact, the attenuation of the input signal can be selected and thus a range for the recording of the signal can be selected.

FIG. 3 illustrates the circuit of the voltage level detector 29. The remaining voltage level detectors 20-28 are similar to the voltage level detector 29, as shown in FIG. 3, except they do not have an output indicating that the input signal is off scale. As shown in FIG. 3, the output signal of the range selector 11 is applied to an input of a summing amplifier 111. A reference voltage of O.9E is applied to the other input of thc summing amplifier 111. Accordingly, the output of thc summing amplifier 111 will be positive if the output voltage received from the range selector 11 is greater than 09E and will be negative if the output voltage from the range selector is less than 0.9E The output from the summing amplifier 111 is applied to another summing amplifier 113 and to a gate 115. A reference voltage of O.1E is applied to a second input of the summing am plifier 113. Thus, if the output of the summing amplifier 111 is positive, but is less than 0.1E the output of the summing amplifier 113 will be negative, and if the output of the summing amplifier 111 is positive and is greater than 0. lE the output of the summing amplifier 113 will be positive. Accordingly, the output of the summing amplifier 111 will be positive and at the same time, the output of the summing amplifier 113 will be negative only if the output signal from the range selector 11 is between 0.9E and E The gate 115 operates to detect the combination of polarities from the summing amplifiers 111 and 113 and will produce a signal on its output 117 if the output signal from the summing amplifier 111 is positive and at the same time the output signal from the summing amplifier 113 is negative. If the output signal from the summing amplifier 111 is positive and the output signal from the summing amplifier 113 is also positive, this will mean that the output signal from the range selector is greater than E, which will be off scale for the recorder. The gate 115 detects this, and provides an off scale indicating signal on output 119. The off scale signal could be used to automatically change the range in the range selector 11. If the output signal from the range selector 1] is less than 0.9E the output signal from the summing amplifier 111 will be negative, and accordingly, the output signal from the summing amplifier 113 will also be negative. Thus, under these conditions, the gate 115 will not produce an output signal. The remaining voltage level detectors 20-28 are the same as that shown in FIG. 3 except that the reference voltage applied to the summing amplifier 111 is selected to correspond with the lower limit of the voltage increment being detected by the detector. For example, the voltage level detector 25 will have a reference voltage of 0.5E applied to its summing amplifier 111.

As shown in FIG. 4, the E switch comprises a voltage divider 121 comprising 10 equal resisotrs con nected in series from the reference voltage E to ground. Taps are taken off between each adjacent pair of resistors and at ground and connected to the inputs of linear gates -139, which as pointed out above, pass the applied analog voltage to their outputs when enabled. The outputs of the linear gates 113 are connected in common to the output of the E switch. Each of the linear gates is selectively enabled by the output of the corresponding voltage level detector so the E switch will produce an analog output voltage equal to the lower limit of the voltage increment in which the input signal falls as detected by the voltage level detectors 29.

I The invention as described above in its narrower aspects employs a helical bar printer with a plurality of convolutions to achieve the dramatically reduced full scale response time with high resolution and precision. However, the invention in its broader aspects contemplates other dot printing means in place of the helical bar printer in combination with the voltage level detectors or other equivalent means to establish consecutive voltage ranges in which the input signal may fall or to convert the input signal to a digital representation.

Moreover, it is contemplated that the multiconvolution helical bar arrangement might be used to print electrostatically rather than by impact or by pressure. In such an arrangement, the graph would be printed on electrosensitive paper. Instead of hammer blades, electrically conducting marking blades would be employed to coact with an electrically conducting helical bar. The marking blades or the helical bar, or both, would be in light contact with the electrosensitive paper. When a selected blade is energized, the resulting electrostatic action between the blade and the helical bar will print a dot on the electrosensitive paper. The system would select one of the marking blades in response to the analog input in the same manner as the printer described above and the selected blade would be electrically energized at the proper time to print a dot in the appropriate position to represent the input analog signal.

Many other modifications may be made to the above described specific embodiment of the invention without departing from the spirit and scope of the invention, which is defined in the appended claims.

- I claim:

l. A recorder for recording a graph of a variable I I value comprising a plurality of adjacent dot printing means arranged in a row each operable to print a dot in any selected one of adjacent dot positions along a print line extending in the direction of said row, each dot printing means comprising a single dot marker having at least a portion movable to eachof the dot positions corresponding to such dot printing means and being operable to print a dot at any one of said dot positions along said print line by movement of said portion to the selected dot position and actuation of such dot marker to print a dot at the selected dot position, means to establish a plurality of consecutive analog ranges into which variable value may fall, each of said ranges corresponding to a different one of said dot printing means, and control means to select the dot printing means corresponding to the range in which said variable value falls and to actuate the dot marker of the selected dot printing means to print a dot in position alongthe print line thereof selected in accordance with where said variable value falls in the range corresponding to the selected dot print means.

2. A recorder as recited in claim 1, wherein said plurality of dot printing means comprises a helical bar having a plurality of convolutions, and a plurality of marking blades positioned adjacent to said bar, each spanning one of said convolutions, each of said dot markers comprising one of said convolutions and one of said marking blades and being operable to print a dot at the intersection between the blade and said bar when actuated.

3. A recorder as recited'in claim 2, wherein there is I provided means to rotate said helical bar continuously about its axis and wherein said control means comprises timing means to actuate the dot marker of the selected dot printing means at a time interval after said helical bar has passed a predetermined start position corresponding to where said variable value falls in the range corresponding to the selected dot printing means.

4. A recorder as recited in claim 3, wherein said timing means comprises means to generate pulses as said helical bar rotates and means responsive to the number of pulses generated to actuate the dot marker of the selected dot printing means when the number of pulses generated reaches a value corresponding to where said variable value falls in the range corresponding to the selected dot printing means.

5. A recorder as recited in claim 2, wherein each of said marking blades comprises a hammer operable to strike said helical bar to print a dot.

6. A recorder for recording a variable value comprising means to convert said variable value into a digital representation, a plurality of adjacent dot printing means arranged in a row, each operable to print a dot in any selected one of a plurality of adjacent dot positions along a print line extending in the direction of said row, each of said dot printing means corresponding to a different value of said digital representation, each dot printing means comprising a single dot marker having at least a portion thereof movable to each of the dot positions corresponding to such dot printing means and operable to print a dot at any one of said dot positions along said print line by movement of said portion to the selected dot position and actuation of such dot marker to print a dot at the selected dot position, and control means responsive to said digital representation to select the dot printing means corresponding to the digital value of said digital representation and to cause the dot marker of the selected dot printing means to print a dot in a position along the print line thereof selected in accordance with the amount that said variable value varies from thevalue of said digital representation.

7. A recorder as recited in claim 6, wherein said plurality of dot printing means comprises a helical bar having a plurality of convolutions, and a plurality of marking blades positioned adjacent to said bar, each spanning one of said convolutions, each of said dot markers comprising one of said convolutions and one of said marking blades and being operable to print a dot at the intersection of the blade and said bar when actuated.

8. A recorder as recited in claim 7, wherein there is provided means to rotate said helical bar continuously about its axis and wherein said control means comprises timing means to actuate the dot marker of the selected dot printing means at a time interval after said helical bar has passed a predetermined start position corresponding to the amount that said variable value differs from the value of said digital representation. 9. A recorder as recited in claim 8, wherein said timing means comprises means to generate pulses as said helical bar rotates and means responsive to the number of pulses generated to actuate the selected dot printing means when the number of pulses generated reaches a value corresponding to the amount that said variable value differs from the value of said digital representation.

10. A recorder as recited in claim 6, wherein each of said marking blades comprises a hammer operable to strike said helical bar to print a dot.

11. A method of recording a graph of a variable value with a dot printer having a plurality of adjacent dot markers arranged in a row along corresponding segments of a print line, each of said dot markers having at least a portion thereof movable to a plurality of dot positions in the corresponding segment and being operable to print a dot in any selected one of said dot positions along the corresponding segment by motion of said portion to the selected dot position and actuation of such dot marker at the selected position, comprising the steps of converting said variable value into a digital representation comprising a most significant digit having a radix equal to the number of said dot markers, selecting one of said dot markers in accordance with the value of said most significant digit, moving the movable portion of the selected dot marker to a position along the corresponding segment thereof selected in accordance with the difference between the value of said variable value and said most significant digit, and actuating the selected dot marker to print a dot at the selected position.

12. A method of recording a graph of a variable value as recited in claim 1 1, wherein the movable portions of said dot markers are moved to a selected position and actuated to print a dot at the selected position by cyclically scanning the movable portions of said dot markers through the corresponding segments thereof and timing the actuation of the selected dot marker to print a dot at the selected position as the movable portions of the selected dot marker scans past the selected position. 

1. A recorder for recording a graph of a variable value comprising a plurality of adjacent dot printing means arranged in a row each operable to print a dot in any selected one of adjacent dot positions along a print line extending in the direction of said row, each dot printing means comprising a single dot marker having at least a portion movable to each of the dot positions corresponding to such dot printing means and being operable to print a dot at any one of said dot positions along said print line by movement of said portion to the selected dot position and actuation of such dot marker to print a dot at the selected dot position, means to establish a plurality of consecutive analog ranges into which variable value may fall, each of said ranges corresponding to a different one of said dot printing means, and control means to select the dot printing means corresponding to the range in which said variable value falls and to actuate the dot marker of the selected dot printing means to print a dot in position along the print line thereof selected in accordance with where said variable value falls in the range corresponding to the selected dot print means.
 2. A recorder as recited in claim 1, wherein said plurality of dot printing means comprises a helical bar having a plurality of convolutions, and a plurality of marking blades positioned adjacent to said bar, each spanning one of said convolutions, each of said dot markers comprising one of said convolutions and one of said marking blades and being operable to print a dot at the intersection between the blade and said bar when actuated.
 3. A recorder as recited in claim 2, wherein there is provided means to rotate said helical bar continuously about its axis and wherein said control means comprises timing means to actuate the dot marker of the selected dot printing means at a time interval after said helical bar has passed a predetermined start position corresponding to where said variable value falls in the range corresponding to the selected dot printing means.
 4. A recorder as recited in claim 3, wherein said timing means comprises means to generate pulses as said helical bar rotates and means responsive to the number of pulses generated to actuate the dot marker of the selected dot printing means when the number of pulses generated reaches a value corresponding to where said variable value falls in the range corresponding to the selected dot printing means.
 5. A recorder as recited in claim 2, wherein each of said marking blades comprises a hammer operable to strike said helical bar to print a dot.
 6. A recorder for recording a variable value comprising means to convert said variable value into a digital representation, a plurality of adjacent dot printing means arranged in a row, each operable to print a dot in any selected one of a plurality of adjacent dot positions along a print line extending in the direction of said row, each of said dot printing means corresponding to a different value of said digital rePresentation, each dot printing means comprising a single dot marker having at least a portion thereof movable to each of the dot positions corresponding to such dot printing means and operable to print a dot at any one of said dot positions along said print line by movement of said portion to the selected dot position and actuation of such dot marker to print a dot at the selected dot position, and control means responsive to said digital representation to select the dot printing means corresponding to the digital value of said digital representation and to cause the dot marker of the selected dot printing means to print a dot in a position along the print line thereof selected in accordance with the amount that said variable value varies from the value of said digital representation.
 7. A recorder as recited in claim 6, wherein said plurality of dot printing means comprises a helical bar having a plurality of convolutions, and a plurality of marking blades positioned adjacent to said bar, each spanning one of said convolutions, each of said dot markers comprising one of said convolutions and one of said marking blades and being operable to print a dot at the intersection of the blade and said bar when actuated.
 8. A recorder as recited in claim 7, wherein there is provided means to rotate said helical bar continuously about its axis and wherein said control means comprises timing means to actuate the dot marker of the selected dot printing means at a time interval after said helical bar has passed a predetermined start position corresponding to the amount that said variable value differs from the value of said digital representation.
 9. A recorder as recited in claim 8, wherein said timing means comprises means to generate pulses as said helical bar rotates and means responsive to the number of pulses generated to actuate the selected dot printing means when the number of pulses generated reaches a value corresponding to the amount that said variable value differs from the value of said digital representation.
 10. A recorder as recited in claim 6, wherein each of said marking blades comprises a hammer operable to strike said helical bar to print a dot.
 11. A method of recording a graph of a variable value with a dot printer having a plurality of adjacent dot markers arranged in a row along corresponding segments of a print line, each of said dot markers having at least a portion thereof movable to a plurality of dot positions in the corresponding segment and being operable to print a dot in any selected one of said dot positions along the corresponding segment by motion of said portion to the selected dot position and actuation of such dot marker at the selected position, comprising the steps of converting said variable value into a digital representation comprising a most significant digit having a radix equal to the number of said dot markers, selecting one of said dot markers in accordance with the value of said most significant digit, moving the movable portion of the selected dot marker to a position along the corresponding segment thereof selected in accordance with the difference between the value of said variable value and said most significant digit, and actuating the selected dot marker to print a dot at the selected position.
 12. A method of recording a graph of a variable value as recited in claim 11, wherein the movable portions of said dot markers are moved to a selected position and actuated to print a dot at the selected position by cyclically scanning the movable portions of said dot markers through the corresponding segments thereof and timing the actuation of the selected dot marker to print a dot at the selected position as the movable portions of the selected dot marker scans past the selected position. 